1. Field of the Invention
The present invention relates generally to an optical system, and more particularly, to an image forming apparatus including a plurality of scanning type optical devices which deflect and scan an optical beam with a rotating polygonal mirror and a control method of the image forming apparatus.
2. Description of the Related Art
Generally, an image forming apparatus that uses a plurality of image carrier bodies to form a multi-color image accomplishes this by overlapping toner images of different colors formed on the respective image carrier bodies. When forming a latent image to be developed into a toner image on each image carrier body, a plurality of exposure apparatuses deflect and scan an optical beam on each image carrier body using a polygonal mirror.
Image quality in a multi-color image forming apparatus is affected by so-called color misalignment or color misregistration. A color misalignment occurs when a forming position of a toner image of each color deviates from a reference position. For example, when unevenness (hereinafter referred to as a jitter) occurs in the rotating speed of a scanner motor for driving a polygonal mirror, a fluctuation in magnification occurs, causing a color misalignment in a main scanning direction. The color misalignment is less noticeable on a scanning line writing-side (scanning start side) and more noticeable on an image end-side (scanning end-side).
Jitters include “short cycle jitters” with relatively short cycles and “long cycle jitters” with relatively long cycles. A short cycle jitter is primarily caused by variations in the accuracy of surface of a reflecting surface of a polygonal mirror and residual errors in rotational control of a scanner motor. Meanwhile, a long cycle jitter is primarily caused by residual errors in control and air current variations in association with the rotation of the polygonal mirror.
FIG. 12 is a diagram for describing the relationship between a signal waveform of a jitter component having a relatively large amplitude among jitters of a scanner motor and a color misalignment in a main scanning direction. In this case, it is assumed that four respectively independent exposure apparatuses corresponding to black, cyan, magenta, and yellow are to be used. Since each exposure apparatus has a different exposure start timing and performs rotating speed control independently, the phases of the jitter components of the respective exposure apparatuses are different. Consequently, a deviation in the exposure position (image forming position) occurs at an image end side of each exposure apparatus, which in turn causes a color misalignment. An image forming device that renders different colors by overlapping a plurality of different-colored toners assumes that the formation positions of the toner images in the respective colors used in the superposition are consistent. Therefore, unless the formation positions of the toner images in the respective colors used in the superposition are consistent, not only is it impossible to render an intended color but a color misalignment occurs in which, for example, a used toner color becomes undesirably prominent.
Japanese Patent Laid-Open No. 07-151987 proposes an invention that reduces long cycle jitters by covering the periphery of a polygonal mirror with a cylindrical cover to suppress crosscurrent vortex.
The invention according to Japanese Patent Laid-Open No. 07-151987 is truly remarkable in that long cycle jitters attributable to crosscurrent vortex can be reduced. However, there is a limit as to how much air current can be rectified. The distance between a cylindrical guide section that rectifies air current and the polygonal mirror is shortest at edge portions and longest at the center of adjacent edges. Therefore, air pulsation is unavoidable. Although it is possible, in principle, to eliminate air pulsation by vacuumizing the periphery, creating a vacuum state is extremely difficult from both cost and technical perspectives.